1. Field of the Invention
The present invention relates to electronic components fabricated using ceramic substrates, such as multi-layer ceramic devices. More particularly, the present invention relates to electronic components having ceramic layers that incorporate ceramic-metal composite materials and methods of producing such components.
2. Description of Related Art
Multi-layer ceramic (MLC) devices are fabricated from a plurality of stacked ceramic sheets (substrates), wherein each sheet has a particular screened metal pattern on its planar surface and a selected pattern of feed-through vias, or holes, formed through the sheet. The vias in the ceramic sheets are usually filled in the unfired or xe2x80x9cgreenxe2x80x9d state with a slurry of refractory metal paste, i.e., tungsten particles suspended in an organic vehicle. Screen printing techniques, through either a stencil or a screen, are typically used to apply the planar metallization and to force the metallizing paste into the vias. Selected patterned and via-filled sheets are then stacked in the proper order and are laminated together with heat and pressure to form an assembly ready for sintering.
During the sintering operation, the binder is volatilized from the ceramic sheet, the patterned metallization and the via metallization. After binder removal, the temperature is increased to provide densification by sintering of the ceramic and refractory metal portion of the assembly. The metal-filled vias now become electrical conductors and provide selective electrical interconnection between the various layers of the device.
While most vias in an MLC structure are designed to provide electrical interconnection between the multiple layers, vias may also be provided to function as heat sinks for heat generating components that are placed within or on top of the device. Vias can also be included to function as electrical grounds for the components.
One of the drawbacks of screening metals into vias and conductor paths is that the metal pastes represent a significant cost to the manufacturer of the electronic component. Further, metal pastes often contain many volatile organics and other environmentally undesirable constituents that are used to adjust the rheology of the paste. Moreover, the metallized ceramic device must be sintered in a non-oxidizing atmosphere to avoid oxidation of the refractory metal.
Some techniques have been developed to address one or more of these problems. U.S. Pat. No. 4,942,076 by Panicker et al., issued Jul. 17, 1990, discloses a single-layer ceramic substrate with metal-filled vias that is useful for mounting high-frequency hybrid microcircuits. Panicker et al. use a screening process to place a refractory metal (e.g., tungsten) and binder into vias that are formed in ceramic sheets and then sinter the sheets such that the tungsten-filled vias have 10 to 20 percent porosity. Copper is then screened onto the porous tungsten-filled vias and the sheets are heated to reflow the copper into the tungsten. The result is a ceramic substrate having a plurality of vias filled with about 85% tungsten and about 15% copper. It is disclosed that the vias are useful for ground connections and for carrying away internally generated heat. The substrate must be sintered in a reducing atmosphere during both the sintering step and the copper metal reflow step.
U.S. Pat. No. 5,089,881 by Panicker issued on Feb. 18, 1992. Panicker discloses a ceramic-based chip carrier for mounting an IC chip wherein certain vias in the ceramic substrate include a porous refractory metal that has copper metal reflowed into the porosity of the refractory metal. In the process for making the chip carrier, a sintered ceramic substrate is laser-drilled to form vias therein and the vias are thereafter filled with a refractory metal paste and fired in a reducing atmosphere to sinter the refractory metal into a porous mass. Copper is then screened onto the refractory metal-filled vias and is then heated in a reducing atmosphere to reflow the copper into the porous metal. At least three furnace cycles are required and the metallization must be sintered in a reducing atmosphere. This process is also not a co-fire process wherein the refractory metal and the ceramic are sintered in a single step.
U.S. Pat. No. 5,163,499 by Newkirk et al., issued on Nov. 17, 1992. Newkirk et al. disclose a method of forming electronic packages by spontaneously infiltrating a permeable mass of filler material or a preform with molten matrix metal and bonding the infiltrated material to a second material, such a ceramic or metal. Prior to infiltration the filler material or preform is in contact with a portion of the second material such that after infiltration, the infiltrated material is bonded to the second material to form a sealed electronic package.
There exists a need for a method that is useful for forming high-conductivity vias, conductor paths, ground planes and other components of ceramic-based packages that include a highly conductive material, such as copper. It would be advantageous if vias could be easily formed having a high conductivity metal and if the vias had a thermal expansion that was substantially similar to the surrounding ceramic body. It would be advantageous if the components could be sintered in a non-reducing atmosphere. It would also be advantageous if a co-fire process could be used to form the components.
According to one aspect of the present invention, a substrate for a hybrid micro-circuit is provided comprising a thin ceramic sheet and a plurality of vias in the thin ceramic sheet, wherein the vias comprise a porous ceramic matrix which has been infiltrated with a highly conductive metal. The conductive metal is preferably selected from copper and copper alloys.
According to another embodiment of the present invention, a method for making a carrier for electronic components is provided that includes the steps of forming a substantially flat and thin sheet of ceramic, forming a plurality of vias in the ceramic sheet, filling at least a portion of the vias with a ceramic-containing paste capable of forming a porous sintered ceramic, sintering the sheet of ceramic to form a sintered sheet having a plurality of vias comprising porous ceramic with an open porosity of at least about 10 volume percent and contacting the ceramic sheet with a molten metal such that the molten metal infiltrates the porous ceramic vias and fills the open porosity. In one embodiment, the ceramic via is a thermal via.
According to another embodiment of the present invention a substrate useful for mounting electronic components is provided comprising a flat ceramic sheet and a layer adhered at least to a portion of the lower surface of the ceramic sheet that consists essentially of a ceramic-metal composite. The metal infiltrated lower portion provides a heat sink for the mounted electronic components.
According to another embodiment of the present invention a method for forming a metal plane on an electronic substrate is provided by forming a substantially flat ceramic sheet, forming a porous ceramic layer on at least a portion of the ceramic sheet, contacting the porous ceramic layer with a molten metal to infiltrate the metal into the porous ceramic layer and cooling the infiltrated porous layer to form a ceramic sheet having a metal plane adhered thereto.